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close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
close this folder6. Direct current machines
close this folder6.5. Circuit engineering and operational features of customary direct current motors
View the document6.5.1. Direct current motor with permanent excitation
View the document6.5.2. Direct current series motor
View the document6.5.3. Direct current shunt motor

6.5.2. Direct current series motor


The direct current series motor is a direct current motor whose exciter windings (D1, D2) have been series-switched to the rotor winding.

Figure 107 - Circuitry and terminal board of a direct current series motor

1 Clockwise, 2 Anti-clockwise

Dependence of current take-up on the torque (load)

Current take-up by the motor depends on the load. As curve I = f (M) indicates, during idling the motor only takes up minimal current. Current take-up increases through greater load. Thereby, however, the increase in current intensity is greater than the load growth.

Figure 108 - I = f (M); Dependence of current take-up of a direct current series motor on torque

1 Rated current, 2 Rated torque

Dependence of speed on the torque (load)

The speed-torque curve (Figure 109) shows that the speed depends to a considerable extent on the load. Whilst idling speed assumes greater values. Given reduction the motor may “race” under certain circumstances.

Figure 109 - n = f (M); Speed dependence on the torque of a direct current series motor

1 Rated speed, 2 Rated torque

The considerable centrifugal power which then arises can destroy the motor. Therefore the motor must be securely attached to the drive machine. Speed declines markedly as the load increases. The direct current series motor develops a considerable initial torque during starting. It can, therefore, also start given excessive load.

Speed control

Speed can be controlled by

- a series resistor
- a strain field actuator parallel to the exciter winding and by
- changing mains voltage.


Direct current series motors are used where considerable speed ranges and excessive torques are in evidence and “racing” is not possible (e.g. for driving electric railways, cranes, escalators).